The third series of Merlin begins on Saturday – which makes it the perfect time to reflect on the astonishing success of this irresistibly fun but nonetheless ridiculous modern-day medieval television series.

Morgana, once so pure and kind, is morphing into an evil, Celtic witch-queen version of Andrea from The Corrs, all flashing eyes under braided hair. Uther Pendragon is old and in the way, a once-powerful patriarch beginning to look distinctly doddery. His physician, Gaius, is like one of the ageing hippies who sell crystals in the Green Field at Glastonbury. Guinevere resembles a nice girl from Croydon, and Merlin is the downtrodden but clever grammar school boy to Prince Arthur's public school toff, forever hiding his talents under a cloak of servitude in fealty to his slightly dim but astonishingly confident highborn superior.

Herein lies the secret of Merlin's success. It may feature cavernous stone castles, lengthy sword battles and talking dragons, but it also mirrors 21st century British society. Even the thrust of the story is shaped by basic modern fears: mysterious and untameable forces that threaten the safety, stability and, most of all, familiarity of life inside Camelot – a metaphor for England itself. "Every generation has a retelling of the Arthurian legends to suit its needs," says Kate McGrath, who plays Morgana. "I was familiar with the story before we started filming, but I liked the way it was retold for today. Legends grow for the people. That's really their purpose."

As for Angel Coulby, who plays the blameless Guinevere, she came to the series with a level of naivety suitable for her character. "I had seen The Sword In The Stone — the Disney version," she offers. "And I like that film Labyrinth." It was the idea of playing Guinevere as a girl for the masses that appealed, Coulby says. "That's rarely the way she's portrayed, but I based her on my own character so that's how she came out. You need a vivid imagination to put yourself into the fantasy situation of the story."

Hard as it may be to believe, Merlin has actually evolved into being a tad more authentic. When the first series aired, Prince Arthur described things as "cool" – not a word readily associated with sixth century Britain. While not quite mirroring the definitive account of Arthur's life that was completed by Geoffrey Of Monmouth in 1138, the third series does avoid such obvious updates, while still capturing the concerns of the modern age. Kate McGrath even sees Morgana's transformation as a triumph of proto-feminist idealism. "She's not a victim anymore," says McGrath. "And I want to her to still be beautiful and elegant, even if she is going to be a villainess."